Use strict mode for Container Linux Configs

* Enable terraform-provider-ct `strict` mode for parsing
Container Linux Configs and snippets
* Fix Container Linux Config systemd unit syntax `enable`
(old) to `enabled`
* Align with Fedora CoreOS which uses strict mode already

digital-ocean/fedora-coreos/kubernetes/versions.tf

@@ -4,7 +4,7 @@ terraform {
   required_version = "~> 0.12.6"
   required_providers {
     digitalocean = "~> 1.3"
-    ct           = "~> 0.3"
+    ct           = "~> 0.4"
     template     = "~> 2.1"
     null         = "~> 2.1"
   }